Humans are exposed to mutagenic agents principally as a result of occupation and life-style. Chronic exposure to low levels of such agents is, no doubt, much more common than acute exposure, although there are now many documented cases of exposure to moderate/high levels of mutagens, especially in or near the work place. Because practical methods to measure frequencies of mutants induced in vivo have not been available, indirect measurements, such as, determining the frequency of chromosome damage, or sister chromatid exchanges or the induction of tumors are often used. However, these methods are limited in applicability because of the relatively low number of cells examined in chromosome-chromatid studies and the long latent period required for cancer. It is now possible, using the T-cell growth factor, Interleukin 2, to measure the frequency of thioguanine (TG)-resistant T lymphocytes in peripheral blood as a method of assessing if humans exposed to environmental agents show an abnormal level of such cells. However, to maximize the usefulness of such an assay, and interpret the data, it will be important to calibrate the system using T lymphocytes from persons who have been exposed to known concentrations of mutagens and comparing the frequencies with those induced in vitro under controlled conditions, using the same concentrations of the mutagens. Cancer chemotherapy patients undergoing treatment will be used as a source of T lymphocytes for the mutagen-exposed population, since this is one of the few groups of individuals exposed to known levels of mutagenic agents. We propose to develop a quantitative in vitro assay for measuring the increase in frequency of TG-resistant T lymphocytes induced in vitro by the chemotherapeutic agents. Parameters such as, time of exposure, length of phenotypic expression, concentration of selective agent, cell density during selection, etc. will be worked out using well-studied mutagens, e.g., ethylnitrosourea, and peripheral blood lymphocytes from random blood donors. The latter cells will also be used to establish the range of background frequencies. The assay will then be applied to chemotherapeutic agents, as well as to representatives from several classes of environmental mutagens. Information from the in vitro experiments can be used to interpret the in vivo response of T lymphocytes in individuals exposed to various environmental agents, as well as provide data on mechanisms of mutagenesis. Once developed, the in vitro assay can be especially useful for studying the mutagenic effect of very low doses or very weak mutagens. This is because the lack of metabolic cooperation between T lymphocytes allows selection of large numbers of cells at high densities in microtiter plates and the number of target cells available from blood donors is very large.